Last summer I papered my hall and stairs. Recently, however, I noticed some dark patches under an area near the front door. I have stripped some of the paper off to reveal what appears to be a black fungal growth which wipes off easily enough with a damp soapy sponge. Is there anything with which I can prep the wall to make sure the fungus doesn't come back? I'm lucky this time because I have enough paper left over to re-do this bit. However, I don't want to have to do the whole lot if the fungus appears again.

Hi Bahny,
Mould/fungus appears most of the time through bad ventalation, check any air vents you may have are clear.If you have windows frames with the open and close vents at the top open them around the house more often,ventaltion around the house is the best way to avoid condensation.

I'd get some Dettox mould and mildew cleaner to use on the area before papering.

If you have bad ventalation in the house,more than likely it will reappear somewhere else,so open the windows more often and check any air vents you may have are clear.If your house had a previous owner you'd be surprised what some people stuff in them to stop drafts when they are there for ventalation.

Mould growth under wallpaper can also be caused by not using a paste with a fungicide in it. Heavy or non porus papers such as Vinyl must be applied using a paste with fungicide in it. The reason is that water is trapped behind the paper and will cause mould without the fungicide.
With thinner papers the water will soak through the paper and the wall will dry out.

Even if this is not your problem, it is worth considering before you re-paper.

Hi Kenj, they probably did think of that but these days it would be hard to buy a paste that didn't have a fungicide in it. From the original question I would have thought the problem was there before the decoration but not noticed as it was summer time and it would have been like a fine dust as it would have been dry. Then in the winter it becomes active again with the dampness in the air. This is often a problem close to a door or window. Hopefully the answers given will help Bahny cure it.